One type of prior art image sensor is a charge-coupled device (CCD) image sensor. CCD image sensors typically include a very closely packed array of polysilicon electrodes that are formed on the surface of the CCD imaging chip. Conceptually, a CCD is a two-dimensional array of Metal Oxide Semiconductor (MOS) capacitors that collect and transfer photon-generated charge. CCD image sensors typically include multiple shift registers to transfer charge from the CCD array to amplifiers.
Another type of prior art image sensor is an active pixel sensor (APS) image sensor. APS image sensors are typically fabricated using Complimentary Metal Oxide Silicon (CMOS) processing technology, and are also typically referred to as CMOS image sensors. APS image sensors sense light by converting incident light (photons) into electronic charge (electrons) by a photo-conversion process. Color APS image sensors are typically made by coating each individual pixel with a filter color (e.g., red, green, and blue). APS image sensors typically include a photo sensor (e.g., photo diode) and several CMOS transistors for each pixel. Some APS image sensors provide integrated analog-to-digital conversion and full timing control on a single integrated circuit.
Some APS image sensors support sub-sampling modes of operation. In sub-sampling modes, the amount of data that is imaged is reduced, while the field of view is maintained. For example, in a two-to-one sub-sampling mode (e.g., sample two pixels, skip two pixels, sample two pixels, etc.), the amount of image data that is processed is reduced by a factor of four when sub-sampling in both the horizontal and vertical directions. In a four-to-one sub-sampling mode (e.g., sample two pixels, skip six pixels, sample two pixels, etc.), the amount of image data that is processed is reduced by a factor of sixteen when sub-sampling in both the horizontal and vertical directions. In general, enabling sub-sampling increases the frame rate for a constant clock rate or, alternatively, the clock rate may be proportionally reduced to reduce the frame rate and power. Unfortunately, a drawback of conventional sub-sampling methods is that video information on skipped pixels is discarded resulting in an image with artifacts, particularly in scenes with considerable contrast. In other methods, all pixels are quantized. To remove artifacts and reduce date requirements, averaging of the pixel outputs is performed in the digital domain. This results in a pleasing image, but provides no or minimal savings in power. Averaging in the digital domain can add noise to the final image. Also, prior systems underutilize chip resources and consume needless power.